There are several methods known as biometrics for recognizing or identifying an individual from personal biological characteristics. Some of these methods involve imaging of the face or eye and analyzing the facial features, retinal vascular patterns of the eye, or patterns in the iris of the eye. In recent years there has been a demand for more reliable systems to identify individuals, particularly those persons who desire access to a secured area or system. A common example of such a secured system are automated teller machines which allow authorized users to conduct banking transactions. Many of these systems are used by a wide variety of people. Very often these people demand quick as well as accurate identification.
A technique for accurately identifying individuals using iris recognition is described in U.S. Pat. No. 4,641,349 to Flom et al. and in U.S. Pat. No. 5,291,560 to Daugman. The systems described in these references require clear, well-focused images of the eye. The presence of eyeglasses tends to interfere with good eye images because of reflections on the eyeglasses. Contact lenses may also cause reflections that interfere with eye imaging. However, because contact lenses have a greater curvature than eyeglasses reflections from contact lenses are smaller and less of a problems than reflections from eyeglasses.
Reflections may come from the system's own illumination. In this case, calculations show that the irradiance (illuminance for visible light) at the camera lens from the specular reflection of an illuminator from eyeglasses is on the order of 1000 times greater than the irradiance at the camera of the image of the eye caused by diffuse reflection of the illuminator. A camera viewing the eye must have a combination of lens, aperture, and exposure time that will result in a sufficiently bright image of the eye. Thus, the much brighter specular reflection of the illuminator will saturate the picture elements (pixels) of the camera's image sensor that cover the area of the specular reflection, and all information about the portion of an eye image obscured by this reflection will be lost. Furthermore, the values of pixels surrounding the area of the specular reflection may be corrupted by the saturated pixels in a phenomenon called "blooming". This occurs because the pixels of charge-coupled devices (CCD's), the most common electronic imagers, are not well isolated from one another.
Reflections may also come from ambient illumination, such as bright sunlight. The irradiance generated by such reflections depends on specific ambient conditions, but the power of direct sunlight is comparable to or greater than the power of any safe artificial illuminator, therefore ambient illumination can sometimes cause the same kind of obscuring reflection as the system's own artificial illuminator.
It is possible to ask the subject to remove his or her eyeglasses in order to get a good image of the subject's eye. However, this is potentially annoying, and the subject may refuse to remove the glasses, or avoid using the system. Consequently, there is a need for an imaging system that can obtain useful images of the eye while minimizing the effect of specular reflections without requiring the subject to remove any eyeglasses or contact lenses that may be present.